The present invention relates to one-component moisture curable polyurethane compositions. More particularly, the present invention pertains to one-component moisture curable polyurethane compositions, which exhibit good bonding strength without employing a primer when applied to substrates, such as, for example, glass, plastics, painted boards, painted steel plates, aluminum plates and mortar.
Various polyurethane compositions have been widely used as sealants and adhesives. Two types of polyurethane compositions have been known in the art. One is a two-pack type, which holds polyols and isocyanates separately. The other is a one-component type, which cures by such as, for example, moisture in the air. The one-component type has recently found more applications, since it is easy to handle and does not require the mixing of components at an application site.
One-pack moisture curable polyurethane compositions are described, for example, in U.S. Pat. No. 4,374,237 and U.S. Pat. No. 4,687,533. U.S. Pat. No. 4,374,237 describes a polyurethane sealant containing a prepolymer, which contains at least two silane groups per molecule. The prepolymer is prepared by reacting a urethane prepolymer with a secondary aminosilane. U.S. Pat. No. 4,687,533 describes a polyurethane sealant containing a prepolymer having a pendant alkoxysilane group.
However, such polyurethane sealants have a long curing time. This creates a safety problem when the sealants are used in bonding a windowpane to a vehicle body. Moreover, the sealants have unsatisfactory physical properties as a structural material.
U.S. Pat. No. 5,623,044 describes a polyurethane sealant, which contains a reaction product of secondary aminosilane and a polyisocyanate, such as a trimer of isophorone diisocyanate, a biuret adduct of hexamethylene diisocyanate and polyphenylpolymethylene isocyanate.
However, such polyurethane sealants having these reaction products are not sufficient in primary adhesive strength. Moreover, high content of diisocyanate biuret adduct may give a detrimental effect on physical properties of sealants. This causes a need to employ a primer including a silane coupling agent when a window pane is bonded to a vehicle body.
Japanese Unexamined Patent Publication No. 9-32239 discloses a production method of a siding with tiles where a polyurethane based adhesive of one-component moisture curing type is employed. The adhesive contains not only a urethane prepolymer prepared by reacting a polyol having polyfunctional (such as three functional) groups with polyisocyanate, but also other components, such as silane coupling agents of amino type and glycidyl type.
On the other hand, when a one-component moisture curable polyurethane composition is cured, free isocyanates in a urethane prepolymer react with moisture to generate carbon dioxide gas. The gas causes foams to be formed within a cured material. Particularly, foaming occurs more easily under high temperature and high humidity conditions. Therefore, it may cause a problem, if the composition is to be used for applications, such as sealing materials for constructions and sealants for vehicle bodies, where the composition may be exposed to high temperature and high humidity during a curing period. It will be desirable to provide a polyurethane sealant, which solves above problem and has an excellent bonding strength to substrates such as glass, metal, plastics, and painted steel plates without employing a primer.